Inhibitors of human immunodeficiency virus (HIV) protease have been approved for use in the treatment of HIV infection for several years. A particularly effective and recently approved HIV protease inhibitor is (2S,3S,5S)-21-2,6-dimethylphenoxyacetyl)-amino-3-hydroxy-5-(2-(1-tetrahydropyrimid-2-onyl)-3-methylbutanoyl)amino-1,6-diphenylhexane (also known as lopinavir). 